1. Field of the Invention
The present invention relates to a photovoltaic element array and a method of fabricating the same and, more particularly, to a photovoltaic element array capable of reducing decreases in yield and reliability caused by the formation of a transparent layer and a method of fabricating the same.
2. Description of Related Art
Petroleum and coal are said to bring about anathermal of the earth because they produce carbon dioxide when used, and it cannot be said that atomic power is perfectly secure from radiation when an unforeseen accident occurs or even during a normal operation. Therefore, complete dependence upon these sources as energy sources in the future is very dangerous. On the other hand, a more spread of solar batteries using solar light as an energy source is being expected since they have almost no influence on the terrestrial environment. However, the following problems are preventing a full-scale spread of the solar batteries.
Monocrystalline or polycrystalline silicon is in many instances used as the material for solar light electric power generation. However, solar batteries using these materials require a larger energy and a long time to grow a crystal and complicated steps after that. The result is that a mass-production effect is difficult to achieve and therefore it is difficult to provide inexpensive batteries. This is called a problem A.
To solve this problem A, so-called thin-film semiconductor solar batteries using amorphous silicon (to be referred to as "a-Si" hereinafter) or compound semiconductors such as CdS and CuInSe.sub.2 are being extensively researched and developed. The thin-film semiconductor solar batteries can be formed by using inexpensive substrates such as glass, stainless steel, and aluminum substrates, and only functionally necessary semiconductor layers need to be formed on these substrates. Therefore, the fabrication steps are relatively simple and the amount of semiconductors used is small. Thus the thin-film semiconductor solar batteries have the advantage of being producible at a low cost.
Generally, however, the output voltage of each single element is low in these thin-film semiconductor solar batteries. This leads to a problem B that it is necessary to connect a plurality of elements in series to increase the output voltage for most application purposes.
To solve this problem B, in thin-film semiconductor solar batteries, an array of photovoltaic elements connected in series must be formed on the same substrate. The following three patents can be enumerated as well-known techniques of fabricating this series photovoltaic element array.
(a) U.S. Pat. No. 4,254,386 (filing data: Jun. 13, 1976) has disclosed the structure of a series photovoltaic element array in which divided first electrodes, divided thin-film semiconductor photovoltaic layers (PN junction), and divided second electrodes are formed on an insulating substrate.
(b) U.S. Pat. No. 4,292,092 (filing data: Sep. 29, 1981) has disclosed a method which uses a laser beam as a means for dividing a conductive layer and a semiconductor photovoltaic layer.
(c) U.S. Pat. No. 4,697,041 (filing data: Feb. 10, 1986) has disclosed a method which uses a laser beam as a means for electrically connecting a first electrode and a second electrode.
It is pointed out that series photovoltaic element arrays can be produced at a low cost by the use of these methods since individual layers can be easily divided and photovoltaic elements can be easily connected in series in these methods.
Unfortunately, the techniques disclosed in the above patents (a) to (c) have a problem C that a usable substrate is restricted to an insulating film and this limits the degree of freedom of choice of substrates.
Also, when a thin-film semiconductor is used, a roll substrate can be used if a flexible long substrate disclosed in U.S. Pat. No. 4,369,730 (filing data: Mar. 16, 1981) is used. This is suitable for continuous mass-production of solar batteries.
In principle, an insulating resin film can be used as a flexible substrate. However, a substrate must be heated to at least 200.degree. C. to form a thin-film semiconductor with good characteristics. Accordingly, common resin films are difficult to use because they fuse, deform, or release gases when heated. This is a problem D. Although there is a film such as a polyimide film having a high heat resistance, this film is difficult to process and expensive. This is a problem E.
To solve these problems D and E, thin metal plates such as a stainless steel plate and an aluminum plate are put into practical use as flexible long substrates. Since a metal substrate has conductivity, however, it is necessary to form a series photovoltaic element array after an insulating film is formed on the surface of the substrate.
As a method of solving this problem, U.S. Pat. No. 4,410,558 (filing data: Mar. 16, 1981) has disclosed a technique of fabricating a series photovoltaic element array by forming an anodized layer on an aluminum substrate.
Meanwhile, U.S. Pat. No. 4,419,533 (filing date: Mar. 3, 1982) has disclosed a method of forming a back reflecting layer to increase the efficiency of a photovoltaic element.
Solar light incident through a transparent second electrode is absorbed by a photovoltaic layer to generate electric power. However, there is a problem F that some light components, particularly those having long wavelengths, are not absorbed by the photovoltaic layer. The leakage light from the photovoltaic layer reaches a first electrode. Therefore, a technique of increasing the light utilization is proposed by which a metal with a high reflectance is used as the first electrode, and the leakage light from the photovoltaic layer is returned by this metal layer to the photovoltaic layer and absorbed by it. When a transparent layer is additionally formed between the first electrode layer and the photovoltaic layer, the reflectance of the first electrode is further increased by an optical action. Furthermore, when corrugations are formed at a pitch of the wavelength of light on the surfaces of the first electrode and/or the transparent layer, solar light is confined inside the photovoltaic layer, and this significantly increases the absorption to the photovoltaic layer. The first electrode and the transparent layer having this action are collectively called a back reflecting layer. The back reflecting layer is also effective to a series photovoltaic element array.
Generally, however, the transparent layer is regarded as a part of the first electrode and is patterned to form grooves together with the first electrode. Consequently, a step is raised by the thickness of the transparent layer and this readily causes defects in the photovoltaic layer formed on it. This degrades the yield and reliability of a solar battery. Accordingly, it is considered that the back reflecting layer is practically difficult to apply to a series photovoltaic element array although this is desirable to improve the efficiency.
The details of the problems of a series photovoltaic element array fabricated by forming the transparent layer on the basis of the above viewpoint will be described below.
(1) Defects readily occur in the steps of grooves formed in the transparent layer:
Although the resistivity of the transparent layer is not as low as that of a metal, it is much lower than that of the photovoltaic layer. Therefore, to electrically separate the first electrode, it is necessary to form grooves of the same pattern as the first electrode. However, the transparent layer generally has a thickness of about 0.1 to 1.0 .mu.m. Accordingly, when the thickness is at most about 0.5 .mu.m as in the case of an a-Si photovoltaic layer, poor step coverage occurs to easily cause defects. PA1 Since the transparent layer is transparent to a YAG laser (wavelength=1.06 or 0.53 .mu.m) often used in the processing of solar batteries, a considerable beam intensity is required to form grooves in the transparent layer. When grooves are formed by a laser beam with a high beam intensity, however, the laser beam reaches an insulating layer and damages the insulating layer. This causes short circuits to easily occur between the first electrode and the substrate.
(2) Grooves are difficult to form in the transparent layer by laser processing:
For the reasons (1) and (2) described above, the development of a technique capable of reducing decreases in yield and reliability caused by the formation of the transparent layer is being desired.